Piezoelectric crystal filters are commonly found in analog and digital radio communication devices. Typically, radio devices require filters to have a number of poles depending on the frequency response requirements of the radio. In the design of a radio, for example, the sensitivity and selectivity are specified and the designer incorporates filters having the requisite number of poles to provide the specified sensitivity and selectivity. If, for example, a two-pole filter will not provide the required selectivity, two or more cascaded two-pole monolithic crystal filters may be used.
In addition, the trend in radio devices is towards smaller and lighter construction. Adding two-pole filters in a radio to improve selectivity goes contrary to this trend. There have been some efforts to provide multiple-pole monolithic devices within a single package. However, these devices tend to be harder to produce. Also, additional poles require additional electrical connections to connect to all of the individual resonators of the filter. This requires a larger package or a piezoelectric device with a smaller active area to accommodate the additional electrical connections. Both of these conditions are undesirable.
FIG. 1 shows a prior art three-pole monolithic filter incorporating three resonators. A piezoelectric plate 16 has an upper 18 and a lower surface 20 with three resonators 10,12,14 defined by opposing electrode pairs. An input electrical trace 22 is coupled to a top electrode 30 of a first resonator 10 and a first ground electrical trace 24 is coupled to a bottom electrode 32 of the first resonator 10. An output electrical trace 26 is coupled to a bottom electrode 40 of a last resonator 14 and a second ground electrical trace 28 is coupled to a top electrode 38 of the last resonator 14. A third ground electrical trace 42 is coupled to a top electrode 34 of a middle resonator 12 with a bottom electrode 36 of the middle resonator 12 coupled to the bottom electrodes 32,40 of the first and last resonators 10,14.
As can be seen, the third ground electrical trace 42 requires additional room on the piezoelectric plate 16 to run towards an edge of the plate. As is known in the art, vibrations under the active area (electrodes) of the filter resonators must not be dampened if at all possible. This can only be accomplished by mounting the plate or wirebonding to the electrodes at locations on the plate furthest away from the active area. This has necessitated the use of devices such as the third electrical trace 42 in prior art multi-pole filters. Disadvantageously, this filter and its electrical connections can only be realized by using a larger plate which requires a larger package or by shrinking the active area (electrodes) on the piezoelectric plate which impairs performance.
FIG. 2 shows a second prior art device which is a two-pole filter. This device is similar to the description of the prior art device of FIG. 1, described above. However, this device, instead of having a middle resonator, has narrow shield electrodes 46 between the input and output electrodes 30,40. In addition, this device has the input and output electrical traces 22,26 on different sides of the piezoelectric plate and a fourth ground trace 44 coupled to a bottom shield electrode. Nevertheless, this two-pole filter has the same disadvantage as the three-pole filter of FIG. 1, in that, the third (and fourth) ground electrical trace 42(44) requires additional room on the piezoelectric plate 16.
There is a need for a monolithic crystal filter having three or more poles to provide improved selectivity in a radio without the drawback of using a larger blank or smaller active area to provide room for additional electrical traces.
Accordingly, an ability to provide a multi-pole monolithic crystal filter that can be enclosed in the same size packaging and on the same size piezoelectric blank as existing two-pole filters would be an improvement over the prior art.